1. Technical Field
The present invention relates to a communication system in which data is transmitted and received among a plurality of communication devices. More specifically, the present invention relates to a relay device relaying transmission and reception of data among communication devices, the relay device being able to shorten time required for exchange of data when data is exchanged between a plurality of relay devices while further maintaining the data held by the relay devices to be identical, and also relates to a communication system including the relay device and to a communication method.
2. Description of Related Art
In recent years, a system in which a plurality of communication devices are connected together to mutually exchange data and to perform a wide variety of processing in cooperation with one another while each communication device is assigned a function has been utilized in various fields. In the field of vehicle-mounted LAN (Local Area Network) mounted on vehicles, ECUs (Electronic Control Units) are used as communication devices to cause each ECU to perform specialized processing and exchange data with one another, thereby implementing various functions as a system.
As the function of each communication device is specialized while the number of functions to be performed by each communication device is increased, the number and types of communication devices connected to a communication medium are also increased. Moreover, various functions are expected for the system, which generates the need for each communication device to share data and cooperate with one another, increasing the amount of data to be transmitted.
Increase of the amount of data transmitted and received through a communication line causes delay or loss of data as a result of collision. Significant delay or loss of data may be fatal to the driver assistant function such as brake control by ECU.
It is therefore general to employ a structure in which a plurality of communication lines are used while ECU is connected to each of the different communication lines. This is to avoid wasteful use of communication lines by collecting ECUs that share data in common. Moreover, to effectively use the communication lines to address the increase in the types of ECUs, such a structure may also be employed that ECUs are separated by the types of data to be transmitted and received, and connected to the communication lines with different communication speed. In these structures, different communication lines are connected with each other by a relay device that controls transmission and reception of data.
Even if the communication devices are divided into groups, the amount of data transmitted to communication lines is not reduced with the structure in which all the data required for control of each communication device is transferred via a relay device. Therefore, such a technique has been disclosed in Japanese Patent Application Laid-open No. 2007-300331 that the data received from each communication device is once stored in a database from which the data required for each communication is appropriately read out to be transmitted, and the data stored in a database is transmitted and received among devices each having a database to share the database.